Generally, a small sewage treatment and disposal installation includes a primary station, comprising a facility (e.g a septic tank) for procuring digestion-type reactions, usually of an anaerobic character. The water emerges from the primary station with its dissolved nitrogen content primarily now in the form of ammonium.
This ammonium-laden water is collected (e.g in a pipe) and is pumped or otherwise transferred to a (usually-separate) nitrification station. The nitrification station may also be referred to as an aeration station. Here, the water is exposed to air, and the dissolved ammonium (NH4+) is oxidized and transformed into dissolved nitrate (NO3−). Also, some or most of the dissolved carbonaceous-BOD content of the water entering the nitrification station is oxidized and transformed into carbon dioxide.
Traditionally, the aeration station has comprised a tile-bed soakaway, which serves the function, firstly, of providing and ensuring good exposure of the ammonium-laden water to atmospheric air; and secondly, of performing the mechanical function of infiltrating the water into the ground in such manner as not to erode or damage the ground formation, over a service life of many years. These two functions can be regarded as separate, functionally, in that the tile-bed comprises not only the nitrification station but comprises also the water infiltration or injection or disposal station. In many installations, in fact, the disposal station is also separated physically from the aeration station.
If/when it is desired or required to remove the nitrate from the water, a further station is needed, that being a de-nitrification station. Adding a de-nitrification station can be difficult in the traditional tile-bed soakaway system, because the water has to be intercepted, and the de-nitrification station has to be installed, downstream of the tile-bed, i.e after the nitrified water is already in the ground. U.S. Pat. No. 5,318,699 shows one way in which a de-nitrification station has been incorporated into a traditional septic-based sewage treatment system. It will be understood that it would often not be economically feasible to add such a de-nitrification station underneath an existing aeration/nitrification station and disposal station.
Digging a downstream trench to intercept the water can sometimes be done, if the lay of the land enables this to be cost-effective. This can be done especially when the nitrate-laden water is agricultural run-off, and the trench is a drainage ditch alongside the field.
The de-nitrification reaction, i.e. the conversion of dissolved nitrate to a more acceptable form of nitrogen such as nitrogen gas, is a reduction reaction, and requires anoxic conditions. Sometimes, this can be simple; for example, given the presence of a trench or ditch, in that case it might be easy enough to arrange for the reduction reaction to take place underwater, i.e submerged, thereby procuring the required anoxic conditions.
However, generally, in the traditional arrangement in which the aeration station serves also as the disposal station, it is not economically feasible to collect and extract water that is already in the ground, to run that water through a de-nitrification station, and then to put the de-nitrified water back into the ground. Rather, the addition of the de-nitrification station to an existing system will usually only be economical if, in the existing system, the nitrate-laden water is actually contained in, i.e is conveyed in, a conduit such as a pipe. It is recognized that, when the nitrate-laden water is indeed conveyed in a pipe, it is usually easy enough for the designer to arrange the pipe as the inlet for the de-nitrification station.
The de-nitrified water emerging from the de-nitrification station can then be piped or otherwise conveyed to the disposal station—which might include a soakaway of some kind—wherein the water is infiltrated into the ground, discharged into a river or other body of water, etc.
Thus, in an existing installation, it is a simple matter to add a de-nitrification station if the effluent water from the aeration or nitrification station is conveyed in a conduit such as a pipe. Equally, in a new sewage treatment installation, when the installation is to include a de-nitrification station, the designer should see to it that the nitrate-laden effluent water from the nitrification station is conveyed in and contained in a pipe or conduit, so that it is a simple matter to position the de-nitrification station between the nitrification station and the disposal station.
The de-nitrification processes and reactions are micro-biological, and the de-nitrification station should be so engineered as to procure the conditions required to ensure viability of the colonies of appropriate anaerobic bacteria, which can utilize nitrate instead of oxygen.
The de-nitrification station should be airtight, and sealed off from atmospheric oxygen. As mentioned, traditional designers of de-nitrification stations have preferred to procure the required exclusion of oxygen by submerging the de-nitrification station underwater. Conventional de-nitrifiers have relied on excluding oxygen by submergence of the treatment medium in the water being treated, in, for example: municipal sewage treatment plants; in small (septic-tank-based) sewage treatment installations; and in agricultural run-off facilities.
Also, a source of organic material (i.e carbon) is required for the micro-biological de-nitrification station. In one kind of conventional de-nitrifier, the carbon source has been e.g wood chippings, or the like. In this case, the anaerobic microbe colonies establish themselves on the wood chippings matrix, whereby the matrix within which the bacteria reside is itself consumed by the bacteria—which means that the source has to be replaced after a period of time. It is also conventional to provide the carbon in liquid form, to be injected periodically (or continuously) into the water being treated, and at the same time to provide a matrix of an inert (i.e non-biodegradable) matrix. The systems as described herein utilize carbon in liquid form, which is added as required, and utilize a non-biodegradable matrix or filter medium.
For the purposes of this specification, a “free-draining” body of treatment material is contrasted with a “submerged” body, in that, in a submerged body, the whole body of treatment material remains permanently submerged, throughout operation. In a submerged system, any portion of the treatment material that might lie out of (i.e above) the water would not contribute to treatment.
In a free-draining body of treatment material, by contrast, the water being treated is applied to the top of body or heap of treatment material e.g by being sprinkled or sprayed on top of the body or heap. The body is not submerged in water. In a “free draining” system, however, it is not ruled out that there can be a level or pool of water in the bottom of the container, and that a small portion of the treatment material might be permanently submerged in that pool of water.
Whether a body of treatment material is free-draining or submerged is determined, in many cases, by the height of the water-outlet-port. Basically, if the outlet-port is located below the body, the body is free-draining; if the outlet-port is located above the body, the body is submerged. (However, in some submerged configurations, the level of the water is not determined by height of the outlet-port.) A body of treatment material is said to be “free-draining” if, when water is applied on top of the body, the water travels downwards through the body, under gravity, and then emerges from the bottom of the body.
In the case of a water-outlet-port being located part-way up the height of the body of treatment material, the upper part of the body would be free-draining, and the lower part would be submerged. For present purposes, the free-draining upper part of the body has to be substantial—that is to say, has to be large enough to make a substantial contribution to the water treatment taking place in the body.
Water to be treated can be applied to the body of treatment material in periodic intermittent doses; or alternatively the water can be applied in a continuous stream. The expression “free-draining”, as applied to dosed application, does not necessarily mean that the body of material dries out between doses; for example, in the systems described herein, a good deal of water is held up, by capillary action, in the body of absorbent treatment material, between dosings.